2 Samuel 13:11

And when she had brought them unto him to eat, he took hold of her, and said unto her, Come lie with me, my sister.

The words capture Amnon taking hold of his sister Tamar to lie with her, the opening move of his shameful, unnatural crime against her. That act broke the law pronouncing death upon the adulterer, and because it was the deed of the first-born son it made his guilt double.

David, self-condemned by his own sin, let the deed pass unpunished and unrebuked and failed to bring the offender to justice. Absalom, as the natural protector of a sister so foully wronged, took upon himself to avenge her by having Amnon killed. He hid his purpose of revenge for two full years in order to strike more surely at last, then had the drunken, incestuous Amnon slain at a feast of the king's sons by his command.

The crime stood among the bitter fruits of David's own wrongdoing, continually reminding him of his injustice to Uriah and Bath-sheba. In Amnon's sin and in his violent death, twofold judgment was already meted out to David under his own sentence.